Environmentally accelerated degradation and equipment failure are major problems in the maintenance of aerospace systems. Airborne agents, such as moisture (as rain, snow, condensation, humidity) pollutants (sulfur dioxide, nitrogen oxides, ozone, sand, dust, others, particularly wind, and solar radiation) are generally recognized as the proximate causes of such degradation. Most of these, except solar radiation, are most damaging to aircraft on or near the ground, where military aircraft spend the bulk of their time as contrasted with commercial aircraft. Operational factors, conditions relative to aircraft utilization, such as flying hours, payload, altitude, mission profile, pilot skill and general tidiness of flight crews also contribute to accelerated degradation of aircraft. Other contributing factors are maintenance, accidents and design problems.
The term environmental degradation as used herein includes a variety of problems, including corrosion, stress-corrosion cracking, and more. As used herein, corrosion includes all of the foregoing.
The extent of corrosion damage and the related need for maintenance and repair varies substantially from one aircraft to another. These variations are most visible at depot maintenance where nearly identical aircraft in terms of age, flying hours, and other traditional indices of age are observed to be strikingly different with respect to corrosion.
It has been reported that corrosion maintenance costs the Air Force approximately one billion dollars a year, therefore, there is a need for improvements in corrosion prevention, corrosion detection and repair of corrosion damage. Much of these costs can be directly attributed to corrosion inspection procedures. Corrosion is costly and the most destructive airworthiness-related maintenance problem facing the maintenance of a fleet of a military aircraft. Corrosion is extremely difficult to predict, prevent and detect non-destructively early in its formation. For example, access to corroding areas is frequently difficult or impossible, making conventional, non-destructive evaluation techniques generally inadequate to; detect such corrosion. Thus, costly and potentially destructive structural disassembly is frequently required to detect and evaluate the extent of airframe corrosion. Approximately twelve percent of an aircraft's life is spent in some form of maintenance or inspection procedure, thus, if maintenance and inspection time can be decreased and aircraft availability increased, the effective size of a fleet of aircraft can be correspondingly increased.
In the currently accepted corrosion inspection procedures, visual observation of the part being inspected is heavily relied upon, therefore, if corrosion occurs under paint it is very unlikely that such corrosion will be detected before substantial damage to a part has occurred. Many times corrosion cannot be detected under paint by visual examination until the paint blisters and the part is so corroded that it must be replaced. Additionally, the visual inspection methods currently being used to detect and control corrosion are very subjective and are highly operator dependent.
Infrared, non-destructive testing has been used in the past to inspect workpieces for internal defects and one method of doing so was described in U.S. Pat. No. 3,504,524. The method described in U.S. Pat. No. 3,504,524 involved spraying the workpiece with a vinyl base carrier of a carbon pigment to form a constant, high emissivity surface in the infrared region which is easily removed from the surface being tested. After application of the coating, the test surface is heated with a suitable source of radiant energy and the temperatures of successive spots on the test surface are determined by scanning the coated surface with a radiometer. Output from the radiometer is then transmitted to an oscillioscope or other display device where any flaws in the workpiece are displayed as infrared picture. U.S. Pat. No. 3,504,525 is particularly concerned with controlling the emissivity of the test surface at a standard level by applying a protective coating that has uniform radiating characteristics to the test surface. An essential characteristic of the coating applied to the test surface is that it is easily removed when the test is completed.
U.S. Pat. No. 3,020,745 also discloses the use of an infrared detector to test metal objects for flaws. In this method the area to be inspected is heated by induced eddy currents which uniformly increase the temperature of the test surface in the absence of flaws. At a flaw, the induced current is concentrated about the edges or corner of the flaw and a hot spot develops which is detected by an infrared detector. This method requires that the test surface be covered with a thin homogeneous coating having a high emissivity. U.S. Pat. Nos. 4,037,473; 3,314,293 and 2,846,882 also disclose the use of infrared detectors to measure the temperature of a workpiece. Additionally, the use of infrared thermography as an inspection tool is discussed in "Troubleshooting Products through Infrared Thermography" which appeared in the Nov. 10, 1983 issue of Machine Design. This paper points out that thermography can be used to characterize a complete temperature field around a particular point of interest. It also points out that only surface temperatures can be measured by this technique and that accurate readings are hard to obtain from shiny surfaces.